1. Field of the Invention
The present invention relates to a printhead substrate including electrothermal transducers (heaters) to generate thermal energy for discharging ink. The present invention also relates to a printhead having the printhead substrate assembled therein, and to a printing apparatus configured to print an image by using the printhead.
2. Description of the Related Art
For example, Japanese Patent Laid-Open No. 2002-374163 describes an ink jet printhead in which electrothermal transducers (heaters), a driver, and a logic circuit are formed on a silicon substrate. In accordance with image data, the logic circuit drives the electrothermal transducers through the driver. An arrangement of power supply wirings (lines) for the electrothermal transducers in a substrate (printhead substrate) is described in, e.g., U.S. Pat. No. 6,409,315.
Resolution of a color ink jet printer utilizing the thermal ink jet technique has been increased year by year. Also, in an ink jet printhead used to print an image with high quality, a density at which a plurality of discharge ports (nozzles) are arrayed to discharge ink droplets has been increased from 600 dpi (dots/inch) to 900 dpi and further to 1200 dpi per color of ink. A practical arrangement of such a printhead is described in, e.g., U.S. Pat. No. 6,474,790.
Further, the size of an ink droplet discharged from the printhead has been reduced to lessen graininess in a halftone area of a gray-scale image and an intermediate-tone or highlight area of a photographic color image. In a printhead to discharge color ink, for example, there is a tendency to reduce the droplet size year by year from an ink discharge amount of about 15 pl several years ago to 5 pl and further to 2 pl. Such a printhead is adaptable for a user's needs for higher-quality printing when the user wants to print a graphic or photographic color image with higher quality. However, when the user wants to print a relatively rough image, e.g., a color graph in business forms, which does not require high resolution, a contradiction to the demand for higher-speed printing is resulted because the number of scans necessary for printing is increased due to the reduction of the droplet size.
With the view of solving such a problem, a printhead has been proposed which has both features of higher-quality printing realized with the reduction in size of the discharged ink droplet and a higher-speed printing using the ink droplet of a larger size. For example, U.S. Pat. No. 5,754,201 proposes a method of arranging a plurality of electrothermal transducers in one nozzle and modulating an ink discharge amount. As another example, U.S. Pat. No. 6,137,502 proposes a method of arranging, in one printhead substrate, a plurality of discharge ports (nozzles) which have different ink discharge amounts from each other.
In addition, a method of transferring image data having an increased data quantity to a printhead and driving the printhead in a more compact way is proposed as a time-division driving method in U.S. Pat. No. 6,966,629, for example. With the proposed time-division driving method, a plurality of adjacent electrothermal transducers (heaters) are set as one group and the electrothermal transducers (heaters) in the group are driven successively on the time basis such that plural one of the electrothermal transducers in each group are not driven at the same time.
In recent ink jet printers (ink jet printing apparatuses), as described above, the droplet size of the discharged ink has been reduced to obtain an image with higher quality. On the other hand, a higher printing speed has also been demanded. When the same image is simply formed, the same ink amount is required. For example, when the amount of the ink droplet is reduced to ½ just by decreasing the droplet size of the discharged ink, the printing speed is also reduced to ½. In that case, the reduction of the printing speed can be avoided by discharging the ink in the same amount toward a printing medium. To realize it, however, the number of heaters (electrothermal transducers) arranged per unit length of a nozzle array has to be doubled. If the number of heaters is doubled while the pitch between two adjacent heaters is kept the same, the size of a printhead substrate in which the heaters are arranged is increased by two folds or more. The so-called serial scanning printing apparatus, in particular, suffers from drawbacks of increasing the size of the printhead which is moved in the apparatus at a high speed, making production more difficult, increasing the size of the printing apparatus, and generating larger vibration and noise. To avoid those drawbacks, it is necessary to reduce the heater pitch and to array the heaters at a higher density.
On the other hand, from the viewpoint of stably discharging the ink droplets, a stable voltage has to be applied to the heaters. If all the heaters are driven at the same time, large current flows at a time and a large voltage drop is caused due to the line resistance. As one example of proposals to avoid such a large voltage drop, a time-division driving method is employed which limits the number of heaters driven at the same time and drives the grouped heaters successively at different timings.
Because the reduction in size of the ink droplet is contradictory to obtaining a higher printing speed, it is important to increase the size of the ink droplet so as to realize a higher printing speed in addition to the reduction in size of the discharged ink droplet. In one example of the so-called side-shooter ink jet printhead having discharge ports on the side opposed to the electrothermal transducers (heaters) as described later, the discharge ports and the heaters are arranged corresponding to small ink droplets and large ink droplets, respectively. With the printhead thus constructed, by selectively driving the heaters for the small ink droplets and the heaters for the large ink droplets, the small ink droplets and the large ink droplets can be selectively discharged and higher quality of a printed image and a higher printing speed can be both realized. However, because the number of arranged heaters is increased, it is necessary to reduce the heater pitch and to array the heaters at a higher density.
Thus, in any of the cases of arraying the discharge ports at a higher density to reduce the size of the ink droplet for realizing higher quality of the printed image and the case of selectively using the small ink droplets and the large ink droplets to realize higher image quality and a higher printing speed, the heaters have to be arrayed at a higher density.
Such a requirement can be realized by employing heaters each having a small area to reduce the size of the ink droplet and by arraying the heaters at a higher density corresponding to the printing density. However, when the heaters are arranged in the form of a row at a higher density, rules for a line (wiring) width and a line-to-line distance are decided based on the restriction on the current density in lines associated with the heaters and the rate determined in a production process using photolithography. Accordingly, it is difficult to ensure a heater area for realizing the desired ink discharge amount.